This invention relates to a horn antenna for a main beam with a broad beamwidth in the H-plane of the antenna, the beamwidth varying with frequency substantially less than the usual nominal inverse proportionality to frequency.
More particularly, the invention relates to a horn antenna comprising a horn having an aperture and further comprising means for launching electromagnetic energy into the horn towards the aperture. With reference to a cylindrical co-ordinate system with parameters z, .phi. and r, z representing distance masured parallel to a rectilinear z-axis from a plane normal thereto, .phi. representing an angle measured about the z-axis from a datum and r representing radial distance from the z-axis, the horn has a wide angle of flare about the z-axis in the .phi., r plane. The angle is less than 360 degrees, and the horn is bounded over the whole of the angle of flare by conductive surfaces spaced apart in the z-direction. Since an antenna is reciprocal in nature, the launching means are to be understood to mean additionally or alternatively means for receiving electromagnetic energy propagating in the horn.
Such an antenna may be used in a broad-band direction-finding system comprising a set of N adjacent similar such antennae whose respective main beam axes are spaced at regular angular intervals of (360/N) degrees (normally in azimuth). An R.F. source whose direction relative to the system is to be found may be detected by summing the output signals of all the antennae, and the direction may be established by comparing the magnitudes of the output signals of a suitable pair of adjacent antennae of the set. In order to provide substantially the same probability of detection of an R.F. source for all angles in azimuth and in order to provide optimum accuracy in establishing the direction of the source, it is desirable that the power level of an antenna main beam (relative to its peak level) in a direction corresponding to the main beam axis of an adjacent antenna, i.e. at an angle of .+-.(360/N) degrees to its own main beam axis, should lie approximately in the range of -8 dB to -15 dB over the operating frequency range of the system.
An antenna as set forth in the second paragraph of this specification is disclosed in the Applicants' co-pending U.K. Patent Application 8041126 (G.B. 2090068A). In that antenna, electromagnetic energy is launched into the horn towards the aperture (or mouth) of the horn by a rectangular waveguide having a pair of opposed E-plane ridges. In order to obtain a substantially constant beamwidth over an operating frequency range of 3:1 which includes a band of frequencies immediately above the cut-off frequency of the TE.sub.30 mode, the ridges are spaced along the waveguide from the throat of the horn: the generation of the TE.sub.30 mode by the ridged waveguide is so phased with respect to the horn as to minimize variations of beamwidth with frequency in the band immediately above the TE.sub.30 cut-off frequency. The antenna is suitable for an above-mentioned direction-finding system wherein N=8.